The present invention relates to a process for preparing a collagenic material making it possible to control the rate of in vivo biodegredation of this material.
It relates more particularly to a process for treating a collagenic component making it possible to obtain materials whose stability and mechanical properties can be varied depending on the conditions of said treatment, said materials being suitable for diverse biomedical applications of collagen.
Collagen-based biomaterials are currently used in many applications, having the major advantage of being resorbable. However, depending on their applications, it is necessary to control their biological degradation. This is because the mechanical properties of the implanted collagenic material have to deteriorate progressively and said material must finally be entirely digested over a defined period.
Depending on the applications of these collagen-based biomaterials, the degradation of the latter must in general take place over a time ranging from a few days to a few weeks.
To achieve these objectives, the collagen properties may be modified in several possible ways. Thus, it is known in the art to carry out treatments resulting in the formation of ionic bonds, hydrogen bonds or covalent bonds (Chvapil et al., in International Review of Connective Tissue Research, Vol. 6, ed. D. A. Hall and D. S. Jackson, Academic Press, UK, 1973, 1-61).
The creation of intermolecular links increases the biodegradation time of the collagenic material and the mechanical strength of the collagen fibers, while reducing the water absorptivity, the solubility and the rate of enzymatic degradation of these fibers (Pachence et al., Medical Device and Diagnostic Industry, 1987, 9, 46-55).
Thus, processes have been proposed in the art which allow the collagen to be crosslinked either by physical methods or by chemical methods.
The chemical methods use crosslinking agents such as aldehyde compounds, among which may be mentioned, in particular, formaldehyde, glutaraldehyde, succinaldehyde, glyoxal and acrolein, or else carbodiimides, diisocyanates and azide derivatives (Pachence et al., Medical Device and Diagnostic Industry, 1987, 9, 46-55; Weadock et al., Biomat. Med. Dev. Art. Org., 1983-84, 11, 293-318; BIOETICA and INSERM, FR 2 617 855).
Aldehyde compounds are, to be sure, the most widely used crosslinking agents but they generate potentially cytotoxic biomaterials.
It is desirable to introduce as few chemicals as possible into an implantable biomaterial, since these additives cause complications and regulatory constraints of increasing severity in order to demonstrate the lack of toxicity of such chemicals.
Moreover, a process is known in the prior art for modifying the collagen by forming aldehyde functions within the collagen itself, by oxidative scission using periodic acid or one of its salts, this treatment crosslinking the collagen at neutral or basic pH (M. Tardy and J. L. Tayot, U.S. Pat. No. 4,931,546).
Finally, it has also been proposed in the prior art to modify the properties of the collagen by functionalizing the amino and carboxyl groups of the amino acids that it contains. According to this approach, charge and polarity modifications may thus slow down or accelerate the degradation of collagen (Green et al., Biochem. J., 1953, 154, 181-7; Gustavson, Ark. Kemi., 1961, 55, 541-6).
As regards the physical methods, these include dehydration, aging, heating in the absence of moisture, and irradiation by ultraviolet rays or by beta or gamma rays.
Of these, irradiation treatment with beta or gamma rays is used to sterilize dehydrated collagenic materials, but, in the light of the existing literature, results in materials whose strength cannot easily be predicted.
The various parameters that can influence this type of treatment are not sufficiently well known to allow the quality of the resulting collagenic biomaterials to be controlled, particularly from the point of view of their mechanical strength and their rate of biodegradation (Sintzel et al., Drug Dev. Ind. Pharm., 1997, 23, 857-878).
U.S. Pat. No. 5,035,715 describes irradiation by gamma rays of a substantially moisture-free mixture of collagen and a mineral material, thereby obtaining a certain amount of crosslinking.
Patent EP 0 351 296 describes gamma irradiation of collagen beads, making it possible to increase their density.
Application WO 95/34332 describes the sterilization of a valvular prosthesis made from porcine tissue, and therefore containing collagen, by an electron beam or by X-ray or gamma irradiation. Beta-type electron irradiation of the prosthesis, slightly crosslinked beforehand by a chemical agent, results in less degradation than gamma irradiation. This document does not teach that it is possible to increase the crosslinking and the degradation resistance of such a valve by beta radiation. On the contrary, it teaches that beta radiation has hardly any influence on the crosslinking.
U.S. Pat. No. 5,674,290 describes the sterilization by gamma irradiation of collagenic implants having a high water content, in a sealed envelope transparent to gamma radiation. When this teaching is applied to collagen, the collagen is precrosslinked by a chemical agent. This documents states that, unlike sterilization of a dry collagenic material by gamma irradiation, sterilization of wet collagenic material by gamma radiation only modifies the enzymatic degradability of the material very slightly. The document suggests, erroneously, that sterilization of such a material by an electron beam would be equivalent to sterilization by irradiation by gamma sterilization.
It may therefore be stated that the various known treatments in the prior art allowing certain properties of collagenic materials to be varied either generate undesirable or potential toxicities in the envisioned applications, or are difficult or expensive to implement, or do not allow effective control of the properties of the final material obtained.
It is an objective of the present invention to provide a treatment making it possible to obtain collagenic materials whose rate of degradation in vivo and whose mechanical properties can be varied according to the potential applications of said materials.
It is another objective of the invention to provide a treatment process resulting in a ready-to-use biomaterial by simultaneously carrying out crosslinking and sterilization of the collagenic material.
For this purpose, the subject of the present invention is a process for preparing a crosslinked collagenic material which is biocompatible and nontoxic and has a controlled in vivo rate of biodegradation, characterized in that it comprises subjecting a collagenic component in the wet state to irradiation by beta rays, the collagenic material obtained being sterile and biodegradable over a few days to several weeks.
The subject of the invention is also the aforementioned process characterized in that the collagenic component in the wet state is combined, prior to its irradiation, with a network of collagen fibers, preferably of helical structure.
The invention also relates to materials obtained by the above process.
The invention also relates to a bicomposite which is biocompatible, nontoxic and sterile, has a controlled in vivo rate of biodegradation and is able to be applied by sutures or staples, characterized in that it comprises only, or mainly, two layers intimately associated and crosslinked with interpenetration of the crosslinked networks, one of said layers being formed from a film based on a crosslinked collagenic component and the other from a compacted compress formed from crosslinked collagen fibers rendered insoluble, especially collagen fibers having a helical structure, prepared from collagen dissolved or dispersed in an aqueous solution.
The inventors have discovered, most surprisingly and unpredictably, that the properties of collagenic materials depend on the mode of irradiationxe2x80x94beta or gamma radiation.
In particular, they have discovered that the degree of hydration of the irradiated material itself plays a decisive role in the final properties of the resulting collagenic material.
Specifically, they have found that the results obtained are markedly different according to the nature of the collagenic component treated and according to the irradiation sterilization conditions applied.
Thus, the inventors have presented a treatment allowing both crosslinking and structuring of the collagenic component treated, simultaneously with its sterilization, which, from a wet material substantially free of any complementary crosslinking agent and, preferably, not crosslinked, leads to a ready-to-use material having defined properties, particularly from the standpoint of its mechanical strength and its in vivo degradation time.
According to the invention, the term xe2x80x9cstructuringxe2x80x9d, of the collagenic component refers to the balance between the degree of crosslinking and the degree of hydrolysis of said collagenic component, which results in biomaterials of greater or lesser strength.
According to the invention, the nature of the collagenic component refers in particular to its state (moisture content), its pH or its functionalization, as the case may be.
The collagenic component used in the process of the invention is in the wet state. The expression xe2x80x9cwet statexe2x80x9d is used here to mean a material having a moisture content of greater than 30%, more preferably between 40 and 95%.
By way of comparison, the dry state corresponds to a material whose moisture content is less than 30%, the content preferably being between 5 and 20%.
In this case, the collagen may be present, in the wet state, in the form of a gel or an aqueous solution, or in the partially dehydrated state, in the form of a film, the moisture content being in the latter case close to 30%, unlike the gels and solutions which contain a much larger amount of water.
Whatever its form, the concentration of the collagenic component (solids content) is a minimum of 0.5%, the concentration preferably being greater than 2.5%.
The collagenic component used for the purposes of the invention may consist of or comprise collagen which is either of animal or human origin or is obtained by genetic recombination means. It is preferable to use native collagen, dissolved in acid pH, or else collagen as obtained after a pepsin digestion treatment. In particular, it may be type I bovine collagen or type I or type III human collagen, or else mixtures of the latter in any proportion.
The collagenic component may also consist of or comprise collagen modified by oxidative scission, especially using periodic acid or one of its salts using the technique mentioned above.
It will be briefly recalled that this technique consists in subjecting an acid solution of collagen to the action of periodic acid or one of its salts by mixing it with a solution of this acid or salt with a concentration of between 1M and 10xe2x88x925M, preferably between 5xc3x9710xe2x88x923M and 10xe2x88x921M at a temperature close to room temperature, for a time ranging from 10 minutes to 72 hours.
According to the invention, an aqueous acid solution of collagen whose concentration is between 5 and 50 g/l is used, the concentration preferably being 30 g/l.
This treatment causes scissions in certain constituents of the collagen, namely hydroxylisine and the sugarsxe2x80x94and thus creates reactive sites without causing crosslinking thereof as long as the pH remains acid.
Oxidative scission of the collagen has the function of allowing subsequent moderate crosslinking of the collagenic material but the invention does not exclude the possibility of carrying out this function by other moderate crosslinking means, for example by beta or gamma radiation, or other moderate crosslinking agents, for example chemical agents at sufficiently low and nontoxic doses.
The collagenic component employed according to the invention may also consist of or comprise collagen that has lost, at least partly, its helical structure, especially by heating to a temperature above 37xc2x0 C., preferably between 40 and 50xc2x0 C. for less than an hour.
A final preparation may especially be obtained, which may be likened to gelatin, but the molecular weight of the elementary chains of which is greater than or equal to 100 kDa.
Treating the collagen solution by heating to a temperature of above 37xc2x0 C. results in the gradual loss of the helical structure of collagen, but the invention does not exclude the possibility of carrying out this function by other physical or chemical means, for example by ultrasound or by the addition of chaotropic agents.
The collagenic component may also be formed from or comprise collagen functionalized at the level of the amino and/or carboxyl functional groups of the amino acids, for example by succinylation or methylation, or by the grafting of fatty acids, or any other method known to chemically modify collagen.
The invention also applies to mixtures of the various aforementioned collagenic components in any proportions.
The collagenic component according to the invention may also contain a macromolecular hydrophilic additive.
According to the present invention, the macromolecular hydrophilic additive has a molecular weight advantageously greater than 3 000 daltons.
Examples of this are synthetic hydrophilic polymers advantageously having a molecular weight of between 3 000 and 20 000 daltons. Polyethylene glycol is particularly preferred.
Other examples are polysaccharides, among which may be mentioned starch, dextran and cellulose, which are preferred.
Provision may also be made to use such polysaccharides in the oxidized form by revealing carboxylic functions in these molecules.
Mucopolysaccharides may also be suitable for purposes of the invention, but these are not preferred as their particular animal origin makes them difficult to prepare while meeting the regulatory standards on traceability.
The hydrophilic additive is selected according to various parameters connected especially with its application, such as its price, its harmlessness, its biodegradability and/or its ability to be easily removed, especially via the kidneys, in the case of therapeutic application.
The concentration of the hydrophilic additive is from 2 to 10 times less than that of the collagenic component.
The process for preparing a collagenic material according to the present invention will be described below in more detail.
The process comprises a step in which the collagenic component as defined above is subjected to irradiation by beta rays with variable doses according to the desired mechanical strength of the final biomaterial and to its in vivo rate of biodegradation.
Advantageously, the collagenic component is treated at neutral pH, preferably between 6.5 and 8, for the purpose of favoring the crosslinking reactions and of obtaining a biomaterial which is biocompatible by virtue of the physiological pH.
The collagenic component is crosslinked/structured by irradiation with beta rays At sterilizing doses, advantageously of about 5 to 50 kGrays, preferably between 25 and 35 kGrays.
Under certain conditions, the doses may be reduced, for example down to 5 kGrays, for materials which are already sterile or have a very low degree of contamination, thereby making it possible to lower the degree of crosslinking.
According to the invention, the beta-irradiation treatment applied to a collagenic component in the wet state makes it possible to obtain a material having a high degradation resistance which is thus biodegradable in vivo over several weeks, whereas exposure to gamma radiation results in a biomaterial having a low degradation resistance, which will thus be biodegradable in vivo over a few days.
These results are the opposite of those obtained for a collagenic component in the dry state, for which irradiation by beta rays results in a biomaterial which will degrade over a few days, whereas gamma irradiation results in a biomaterial which will degrade over a few weeks.
According to an alternative way of implementing the invention, the collagenic component intended to be structured by irradiation is combined beforehand with a network of undenatured collagen fibers, which is advantageously in the form of a compacted compress.
This compress may be prepared from native collagen oxidized using periodic acid or one of its salts.
Fibers are formed from the resulting solution and they are then crosslinked by neutralization.
The oxidized collagen fibers, of helical structure, thus crosslinked are freeze-dried and dehydrated, and then compacted to form the compress.
Next, a solution of collagenic component, prepared as indicated above, is deposited on this compacted fibrous collagen compress.
The assembly is advantageously dried and partially rehydrated in order to increase the concentration of the collagenic component.
Next, the irradiation treatment as described above is carried out.
Thus, the aforementioned assembly is structured/crosslinked, resulting in a bicomposite comprising a layer forming a film based on a crosslinked collagenic component combined with a compacted compress of crosslinked collagen fibers with interpenetration of the crosslinked networks.
In general, this process can be applied to woven or nonwoven compresses of collagen fibers advantageously of helical structure.
The collagenic materials obtained by the process of the invention are useful for the prevention of post-operative adhesion and/or the healing of wounds.
Such materials are particularly useful for promoting the healing of skin wounds and surgical wounds. Their biocompatible nature makes them very easily colonizable by the cells of the various types of tissue with which they are brought into contact.
In the case of internal wounds, it has been found that this healing takes place harmoniously without resulting in anarchic fibrous tissue proliferation responsible for post-operative adhesion.
The healing activity of these materials may, of course, be enhanced by the addition of cell differentiation and growth factors.
The materials according to the invention are therefore recommended for rapidly ensuring high-quality healing, as close as possible to the initial anatomy.
The collagenic materials combined with a network of collagen fibers are also useful for wound healing, as indicated above. They have the advantage of being able to be applied by sutures and by staples, given their very high strength resulting from the irradiation with beta rays.
These materials may also be used for tissue or wall replacement (for example for esophageal walls, intestinal walls, etc.) or for filling tissues or walls (in the event of partial ablation of part of a tissue or wall).